1. Field of the Invention
The present invention relates to a thin-film solar cell, particularly to a micro/nanostructure PN junction diode array thin-film solar cell and a method for fabricating the same.
2. Description of the Related Art
Petroleum is going to be exhausted by the end of this century, and the voice for substitute energies has been cried for a long time. Substitute energies include wind power, wave power, and biofuel. Among them, solar energy has relative higher market acceptability, and many nations have been devoted to the development of solar energy. The German Advisory Council on Global Change predicted that solar energy will provide about 60% of the total energy in 2100. Solar energy is generated by the photovoltaic effect, wherein solar energy material directly converts sunlight into electric energy. The crystalline silicon solar cell has been developed for tens of years, and the related technologies thereof have been very mature. Generally, the monocrystalline silicon solar cell has an energy conversion efficiency of as high as about 20%. However, the fabrication cost thereof is too high to popularize solar energy. The topics of solar energy researches would be developing new materials, processes and systems to promote the energy conversion efficiency and reduce the cost of solar energy.
The thin-film solar cell has become an important technical trend because of low cost. The thin-film solar cell adopts a lightweight, flexible and impact-resistant material as the substrate to replace the traditional hard, heavy and thick silicon substrate and creates a new opportunity to exploit solar energy. The flexible solar panel not only has a cheap price but also has diversified applications, such as the applications in BIPV (Building Integrated Photovoltaics), vehicles, boats, portable power supplies (for mobile phones or notebook computers), consumer electronics, and fabrics (e.g. clothes, curtains, sun shelters, and bags). Besides, the flexible substrate can be integrated with a roll-to-roll manufacture process to produce low-cost and large-area solar cells.
Among various types of thin-film solar cells, the amorphous silicon solar cell is a more mature technology. The amorphous solar cell usually has a p-i-n structure, wherein the p layer and n layer are to establish an inner electric field, and the i layer is formed of amorphous silicon. The i layer has a thickness of only 0.2-0.5 μm and absorbs photons of 1.1-1.7 eV, which is different from 1.1 eV photons absorbed by wafer silicon. It is inappropriate for the i layer to have too great a thickness because a thick i layer increases the probability of electron-hole recombination. However, too thin an i layer absorbs insufficient sunlight energy. Therefore, the amorphous solar cell usually adopts a multi-layer stack design to overcome the dilemma. The unsaturated silicon atoms of amorphous silicon will be structurally changed by sunlight radiation. Therefore, the amorphous silicon solar cell has a main intrinsic problem that the performance thereof will decline quickly and obviously after sunlight radiation and has SWE (Staebler-Wronski Effect) of 15-35%. The multi-layer stack design can also offset the lower SWE value. In the fabrication of the amorphous silicon solar cell, a silicon film is formed with a PECVD (Plasma Enhanced Chemical Vapor Deposition) method; the substrate may adopt a stainless steel plate or a plastic material. The fabrication of the amorphous silicon solar cell uses a roll-to-roll process. However, the deposition is very slow. Additionally, the high-quality electrically-conductive glass is very expensive. Thus, the price of the amorphous silicon solar cell is only slightly lower than that of the crystalline solar cell. Although the multi-layer stack design can increase the efficiency of the solar cell, it also increases the cost. The amorphous silicon solar cell has two main weaknesses of low photoelectric conversion efficiency and poor reliability. For the amorphous silicon solar cell, it has an efficiency of 13.5% in laboratories and an efficiency of only 4-8% in commercial applications, which is far below the efficiency of the monocrystalline or polycrystalline silicon solar cell.
The polycrystalline silicon solar cell is the mainstream of the market, and 90% of the market is occupied by the polycrystalline silicon solar cell and the monocrystalline silicon solar cell. Polycrystalline silicon has an energy gap of about 1.12 eV and absorbs light having a wavelength of between 350 to 1100 nm. Polycrystalline silicon has a pretty wide absorption spectrum. However, polycrystalline silicon is a semiconductor material having an indirect energy gap. Therefore, in the electron transition from a valence band to a conduction band, additional dynamic energy must be supplied to phonons so that electrons can jump to the conduction band. As polycrystalline silicon has numerous grain boundaries, polycrystalline silicon is harder to cut than amorphous silicon or monocrystalline silicon.
The performance of the monocrystalline silicon solar cell is similar to that of the polycrystalline silicon solar cell. However, the monocrystalline silicon solar cell is less damaged by solar radiation than the polycrystalline silicon solar cell. With the photoelectric conversion efficiency unchanged, the monocrystalline silicon solar cell has a service life of as long as 20 years on the surface of the earth. At present, US, Germany and Japan all have solar power plants using monocrystalline silicon, and many nations are planning their monocrystalline silicon test solar power plant. The monocrystalline silicon is indeed very important for the development of photovoltaic power generation systems. However, the high price of monocrystalline silicon impairs the application thereof.
Both the solar cells of monocrystalline silicon and polycrystalline silicon consume a lot of silicon material, but refining high-quality silicon material is a very energy-consuming process. Consequently, the silicon material usually has a cost higher than that of the process of fabricating solar cells. Therefore, reducing material consumption is a very important subject for exploiting solar energy.